1. Field of the Invention
The present invention relates to a method of bonding a semiconductor chip that bonds bumps and pads of a semiconductor chip and a substrate by bringing bumps or pads of the semiconductor chip into contact with pads or bumps of the substrate and applying ultrasonic vibration to the semiconductor chip, and also to a bonding apparatus that uses this method of bonding.
2. Related Art
In recent years, when manufacturing semiconductor apparatuses such as semiconductor packages, flip-chip bonding to mount a semiconductor chip on a circuit board has been carried out using a method that brings bumps on the semiconductor chip into contact with pads on the circuit board and applies ultrasonic vibration to the semiconductor chip to bond the bumps on the semiconductor chip and the pads on the circuit board.
It should be noted that there are also cases where bumps are provided on the circuit board and pads are provided on the semiconductor chip.
According to this method of bonding, the bumps and pads are strongly rubbed against each other by the ultrasonic vibration, which causes an alloying reaction to take place between the metal forming the bumps and the metal forming the pads and so results in the bumps and pads being bonded together.
It should be noted that as the combination of materials for the pads and bumps, it is normal to use gold (Au) as the bumps and aluminum (Al) as the pads.
In many cases, the bumps (gold bumps) of the semiconductor chip are formed as so-called “stud bumps”. Stud bumps are formed by first bonding wires to the terminals of the semiconductor chip on which the bumps are to be formed and then breaking off the wires close to the base parts.
In particular, when such stud bumps are bonded to pads using ultrasonic vibration, since the stud bumps are formed in non-uniform and pointed shapes, the stress produced between the bumps and pads tends to be uneven. For this reason, the alloying reaction between the bumps and pads also becomes uneven and so-called “Kirkendall voids” are produced between the bumps and pads, which worsen the bonding and in turn can easily cause a semiconductor apparatus to be defective.
To avoid this problem, before the bumps and pads are bonded together, a technique that levels the bumps to make the shapes of the bumps more even has been conventionally used.
A conventional method of leveling is disclosed in Patent Document 1. A leveling tool with a flat contact surface that contacts all of the stud bumps of a semiconductor chip is placed in contact with the stud bumps and pressed to flatten the tips of the stud bumps, thereby making the shapes of the bumps even (see Patent Document 1, Paragraphs 0009 and 0051, and FIGS. 4, 16, and 18 to 20).
Patent Document 1
Japanese Laid-Open Patent Publication No. 2002-299362 (Paragraphs 0009 and 0051, and FIGS. 4, 16, and 18 to 20)
However, the leveling carried out in a conventional method of bonding a semiconductor chip requires a process that presses a leveling tool onto the bumps of the semiconductor chip, resulting in the problem of the cost and time required when carrying out this process.
Also, although it is desirable to form the bumps in a smooth shape by leveling, if a flat contact surface of a leveling tool is pressed onto the bumps, sharp corners will be formed at the edges of end surfaces of the flattened bumps, so that when the bumps and pads are ultrasonically bonded together, stress will be concentrated at such edges, resulting in insufficient stress in the central parts and, in some cases, problems relating to poor bonding, such as the creation of Kirkendall voids in the central parts of the end surfaces of the bumps.